If you have not already done so, please follow these steps from the iTunes Store Support website (http://www.apple.com/support/itunes/store/buy_troubleshooting/):

"I bought the wrong version of a song.

Be sure to listen to the song preview before you buy a song. Double-click the song's title to listen to its free preview. Make sure your speakers or headphones are set to a comfortable level.

If you have already purchased the incorrect version of the song or video:

1. Locate the item in your Purchase History (itms://phobos.apple.com/purchaseHistory)
2. Click the "Report a Problem" link
3. Select "I purchased the wrong version of this item (Clean vs Explicit)"
4. Enter a description of the issue that you're experiencing in the comments field and hit Submit

Our support agents will review your request and get back to you as quickly as possible."